CMMG AR-15 Resolute 300 Mk4 Upper Receiver Assembly, 350 Legend, 16.1" Barrel, M-LOK Handguard

The CMMG AR-15 Resolute 300 Mk4 upper receiver assembly in 350 Legend gives you a purpose-built hunting upper that threads onto any standard AR-15 lower—no gunsmith required. If you're in a state where straight-walled cartridges are mandated for deer season, or you just want an effective, low-recoil hunting round, the 350 Legend delivers the velocity and energy you need without the kick of heavier calibers like .30-30 or .243 Win. This is a complete upper: bolt carrier group, charging handle, muzzle brake, and dust cover all come installed and zeroed from the factory.

The Mk4 platform pairs a 16.1" 4140CM barrel (1:16 twist) with a billet 6061-T6 aluminum upper receiver for rigidity and durability. The CMMG EML15 M-LOK handguard gives you modern accessory mounting without bulk, and the direct impingement gas system uses a carbine-length gas port and fixed 0.750" gas block for reliable cycling. The included ZEROED muzzle brake (1/2x28 thread) helps tame recoil and aids in spotting shots downrange. Whether you're assembling your first AR or adding a caliber-specific upper to your collection, this is a plug-and-play solution that arrives ready to hunt.

Available in multiple Cerakote finishes, the Mk4 Resolute 300 balances practical field ergonomics with clean aesthetics. It's lightweight at 4.0 lbs and measures 25" end-to-end, fitting standard shooting bags and truck racks. Pair it with a quality lower receiver, ammunition, and glass—and you'll have a versatile, accurate hunting rifle built to last.

Key Features

• Complete upper receiver assembly—bolt carrier group, charging handle, and muzzle brake factory installed and zeroed
• 350 Legend caliber optimized for straight-wall-only hunting states with reduced recoil vs. .30-30 or .243 Win
• 16.1" 4140CM barrel with 1:16 twist for reliable accuracy and velocity
• Direct impingement gas system with carbine-length gas port and fixed gas block for proven reliability
• Billet 6061-T6 aluminum upper receiver for durability and tight tolerances
• CMMG EML15 M-LOK handguard for modern accessory mounting and ergonomic control
• ZEROED ambi charging handle for left or right-hand operation
• 1/2x28 muzzle brake reduces recoil and aids sight-picture recovery
• Multiple Cerakote finish options
• Works with any standard AR-15 lower receiver

Specifications

Frequently Asked Questions

What AR-15 lower receivers are compatible with this upper?

The CMMG Resolute 300 Mk4 upper is designed to work with any standard AR-15 lower receiver, giving you flexibility in choosing your lower based on your budget and preferences.

Does this upper come assembled and ready to use?

Yes. The bolt carrier group, charging handle, muzzle brake, and dust cover are all factory installed. The accessories are factory zeroed, so you can mount it on a lower and begin setup without additional assembly or head-spacing concerns.

Why would I choose 350 Legend for hunting?

The 350 Legend was introduced as a straight-walled cartridge for hunters in states with caliber or cartridge restrictions—particularly common in Midwest deer seasons. It delivers effective power on target with noticeably less recoil than .30-30 or .243 Win, making follow-up shots easier and reducing fatigue during a hunting day.

Can I adjust the gas block on this upper?

No. This upper features a fixed 0.750" non-adjustable gas block tuned for reliable cycling with 350 Legend ammunition. The carbine-length gas port and direct impingement system are balanced for this caliber and configuration.